Foreign Keys and Primary Keys

A key in which it is not a primary key in its own table is called a foreign key. Although the key may be a primary key in another table.
A foreign key is a relationship or link between two tables which ensures that the data stored in a database is consistent.
The foreign key link is set up by matching a column in one table to the primary key column in another table.
The primary key is one of the unique keys defined on a table, but is selected to be the key of first importance. There can be only one primary key on a table.

DVD Code	Title	Cost	Date Out	Date Due
002	Fargo	£2.50	03/09/04	04/09/04
003	American Pie	£2.50	01/09/04	02/09/04
008	Reservoir Dogs	£2.50	04/09/04	06/09/04
011	Notting Hill	£2.50	04/09/04	06/09/04
014	Prime Suspect	£2.00	27/08/04	28/08/04

The Primary Key is DVD Code.

Member Number	Member Name	Telephone Number	DVD Code
1012	Isobel Ringer	293847	003
1034	Jonh Silver	142536	008
1056	Fred Flintstone	817263	011

Whereas the Primary Key here is Member Number, and the Foreign Key is the DVD Code.
These two tables are linked by the DVD Code.